The determination and knowledge of a satellite's orbit at any point in time is of high importance to a satellite operator. The orbit may be derived from position estimations determined by measurements. For example, a geostationary satellite is nominally located (i.e. located according to plan or design) on an assigned longitudinal position on the geosynchronous arc circulating the earth.
Furthermore, a satellite position estimation system allows precise maneuver assessments. Maneuver assessments involve planning and monitoring the impact on the orbit of executed maneuvers, keeping in mind the aim of economically (i.e. sparingly) using the limited amount of fuel onboard a satellite. Maneuvers are notably necessary to keep a geostationary satellite on its assigned longitude. This allows reliable telecommunication reception and transmission via the satellite's non-isotropic antennas. Such maneuvers are necessary since a geostationary orbit is unstable, notably due to the gravitational forces of the Moon and the Sun. Maneuvers are also executed to change the satellite's orbit in a controlled way in order to modify for example its longitudinal position, which is referred to as a satellite drift, as well as its inclination or eccentricity.
In case of a co-location of multiple satellites on a single orbital longitude, a combination of minor longitudinal, inclination and eccentricity separation between the various satellites exists. This scenario is complex and requires continuous and quasi real-time position estimation and orbital determination for each satellite.
Besides geostationary satellites, precise position estimation may be vital and applicable to any type of satellites or spacecraft, whatever their mission type or orbit.
A satellite position may be determined by round trip delay measurements. A round trip delay measurement implies the transmission of a signal from a transmitting ground station to a satellite and back from the satellite to a receiving ground station, and the measurement of the elapsed time between the transmission of the signal from the transmitting ground station and its reception at the receiving ground station. In any of the following methods, the position of each ground station is assumed to be precisely known.
A known method, the so-called trilateration method, involves three ground stations, each able to transmit and receive a reference signal. Typically, each station independently measures the delay between the transmission by itself of a reference signal to the satellite and the reception of the signal back from the satellite after being relayed by the satellite. The set of three stations performing this operation in parallel provides three absolute distance measurements from the three stations to the satellite so that its position is calculable.
Alternatively, the trilateration method can be converted to a pseudo-ranging method. In this method, the round trip delays are not measured independently but jointly between the ground stations such that only one ground station transmits a single reference signal. This first ground station receives the signal back from the satellite. The other stations also receive from the satellite the single reference signal which has being transmitted by the first ground station to the satellite. The distances between the other ground stations and the satellite are therefore calculated indirectly.
The pseudo-ranging method requires a common time reference between the ground stations, whereas the above-described trilateration method does not necessarily require one.
The satellite position estimation may be carried out by solving a three-sphere intersection problem or using an algorithm such as described in D. E. Manolakis: Efficient solution and performance analysis of 3-D position estimation by trilateration, IEEE trans. on Aerospace & Electronic Systems, Vol. 32, No. 4, October 1996, pp 1239-1248.
There is a constant need for improving the systems and methods for estimating the position of a spacecraft, such as a satellite.
[Lexical Note]
Before summarizing the invention, the use of the phrase “and/or” herein is explained.
In each instance, the phrase “and/or” is used to indicate that the terms, features, or clauses joined thereby are to be taken together or individually, thus providing three embodiments enumerated or specified. In other words, with A and B being two terms, features, or clauses, the expression “A and/or B” covers three alternative solutions: “A and B”, “A”, and “B”.
When the expression “A and/or B” is used first and then the expression “the A and/or the B” is used (for instance in a claim, or in a claim and one of its dependent claims), this covers five alternative solutions:                first “A and B” and then “the A and the B”;        first “A and B” and then “the A”;        first “A and B” and then “the B”;        first “A” and then “the A”; and        first “B” and then “the B”.        
Further uses of the phrase “and/or” will be understood in line with these principles, wherein the inconsistent combinations are not covered. For instance, when “A and/or B” is followed by “C and/or D”, each expression covers three alternative solutions, thus covering nine alternative solutions. However, for instance, when “C” is a substitute “a property of the A” and when “D” is a substitute “a property of the B”, it will be understood that “A and/or B” followed by “C and/or D” covers five alternative solutions only.